Pediatric ENT Anatomy and Embryology With Radiology Correlates

1

How does the size and shape of the external auditory canal differ between children and adults?

In adults, the EAC has a near-sigmoid shape with the cartilaginous portion angling posteriorly and superiorly and the bony portion angling anterior inferiorly. Pulling the helix posterosuperiorly straightens the EAC and allows for better visualization of the tympanic membrane. In the infant the EAC is nearly straight. It then elongates and changes shape until approximately 9 years of age when it is nearly adult size.

2

What is a dimeric tympanic membrane?

The tympanic membrane is made up of three layers: an inner membranous layer, a middle fibrous layer that gives rigidity to the membrane, and an outer squamous layer. If a tympanic membrane perforation does not heal with the fibrous layer incorporated, then that newly healed portion has only two layers (dimeric) and results in a thin, floppy segment. This thinner segment is more easily retracted into the middle ear and can affect the conduction of sound to the ossicles.

3

Why are the tympanic membrane and ossicles required for normal hearing?

Sound, as it is presented to us, travels through air while our hearing organs within the inner ear are bathed in fluid. If we attempt to transmit sound from air to fluid there is a 99.9% loss in energy, which is known as an impedance mismatch. The impedance mismatch is overcome by a series of mechanical advantages including a tympanic membrane that is 21 times the size of the stapes footplate and ossicles that create a lever force of 1.3×. Together these overcome the mismatch in impedance and allow for near full transmission of all sound energy into the inner ear.

4

What are the innervations of the tensor tympani and stapedius muscles?

The tensor tympani is derived from the first pharyngeal arch and thus is innervated by a branch of the fifth cranial nerve. The stapedius muscle is derived from the second arch and thus is innervated by a branch from the seventh cranial nerve. The dampening effects of these two muscles can result in a reduced sound transmission of 15 dB.

5

Why is the stapes shaped like a stirrup?

The stapedial artery is transiently present in fetal development, connecting the future external carotid arterial system with the internal carotid system. This vessel goes through the middle ear and the primordial stapes, creating the structure of the stapes known as the obturator foramen. A persistent stapedial artery ( Fig. 48.1 ) is very rare and may be associated with pulsatile tinnitus, conductive hearing loss, and an absent ipsilateral foramen spinosum.

6

What are the two most common congenital abnormalities of the ossicles?

The two most common ossicular abnormalities are a congenitally fixed stapes and incudostapedial discontinuity. Isolated abnormalities of the stapes are more likely to be unilateral, whereas congenital abnormalities of the other ossicles are more likely to be bilateral.

7

Which nerves run through the middle ear?

Jacobson’s nerve is a branch of CN IX and runs across the tympanic promontory innervating the middle ear mucosa and eustachian tube, providing parasympathetic innervation to the parotid gland. Arnold’s nerve is a branch of the vagus nerve that gives sensory innervation to the external auditory canal. This nerve is sometimes stimulated when cleaning the ear and can make a patient cough. The chorda tympani nerve branches from the descending portion of the facial nerve ( Fig. 48.2 ) and runs medial to the malleus before exiting the middle ear through the petrotympanic fissure. Finally, the facial nerve may be dehiscent superior to the oval window or may be positioned within the middle ear in congenitally malformed ears.

8

What are the named segments of the facial nerve that run through the temporal bone and which is the narrowest?

The internal auditory canal segment of the facial nerve is 7 to 8 millimeters in length and runs superior to the cochlear nerve (think of the mnemonic “7-Up/Coke down”). The labyrinthine segment extends from the internal auditory canal to the geniculate ganglia; this is the narrowest segment and most prone to damage secondary to trauma and/or swelling. The tympanic segment runs from the geniculate ganglion to the second genu, running in the medial wall of the tympanic cavity over the round window and below the bulge of the lateral semicircular canal. The final segment is the mastoid or vertical segment ( Fig. 48.2 ).

9

What is the cochleariform process and what is its relationship to the facial nerve?

The cochleariform process is a curved ridge of bone that houses the tendon of the tensor tympani muscle. This ridge of bone is also a good landmark denoting the anterior position of the tympanic portion of the facial nerve.

10

What are the boundaries of the sinus tympani?

The borders of the sinus tympani are formed by the ponticulus superiorly and subiculum inferiorly. This space is difficult to visualize during surgery without the use of a mirror or angled endoscope. Clinically, this area is important during surgery for cholesteatoma, as the cholesteatoma may have grown into the sinus and can be difficult to extract.

11

What is the promontory of the middle ear?

This bulge on the medial surface of the middle ear represents the prominence of the basal turn of the cochlea.

12

What are some commonly described developmental abnormalities of the cochlea and when does developmental arrest occur?

• Cochleovestibular aplasia, formerly known as a Michel deformity (arrest third week): complete absence of cochlear and vestibular structures ( Fig. 48.3 )

  

• Cochlear aplasia (arrest late third week): absent cochlea; normal, dilated or hypoplastic vestibule; and semicircular canals

• Common cavity (arrest fourth week): cochlea and vestibule form a common space ( Fig. 48.4 )

  

• Incomplete partition Type I (arrest fifth week): cystically enlarged cochlea without internal architecture; dilated vestibule; mostly enlarged internal auditory canal

• Cochlear hypoplasia (arrest sixth week): distinctly recognizable separation of cochlear and vestibular structures; small cochlear bud

• Incomplete partition Type II, formerly known as a Mondini deformity (arrest seventh week): cochlea with 1½ turns, cystically dilated middle and apical turn (cystic apex), slightly dilated vestibule ( Fig. 48.5 )